The quality of a time-division-duplex wireless communication system is greatly influenced by a radio frequency (RF) switch. In order to compensate for the undesirable characteristics of the switch (e.g. the on-state resistance and off-state capacitance), prior art adopts a parallel-resonator configuration to enable resonant of inductance and parasitic capacitance, as disclosed in, for example, “A high performance V-band monolithic FET transmit-receive switch” in 1988 IEEE Microwave and Millimeter-wave Monolithic Circuits Symp. Dig., New York, N.Y./USA, Jun. 1988, pp. 99-101; “W-band SPST transistor switches”, IEEE Microwave and Guided Wave Lett., vol. 6, pp. 315-316, Sep. 1996; “A sub-nanosecond resonant-type monolithic T/R switch for millimeter-wave systems applications”, IEEE Trans. On Microwave Theory and Tech., vol. 46, no. 7, pp. 1016-1019, Jul. 1998; and U.S. Pat. No. 7,239,858, entitled “Integrated Switching Device For Routing Radio Frequency Signals”, or adopts an impedance transformation network to switch the resistance and capacitance of the switch, as disclosed in, for example, “Millimeter-wave MMIC single-pole-double-throw passive HEMT switches using impedance transformation networks”, IEEE Trans. Microwave Theory Tech., vol. 51, pp. 1076-1085, Apr. 2003; and U.S. Pat. No. 6,801,108, entitled “A Millimeter-wave Switch Using Impedance Transformation Networks”. However, the above conventional techniques can only compensate for the resistance and capacitance of particular frequencies, but they fail to consider the frequency response of the overall system.
In “Millimeter-wave MMIC passive HEMT switches using traveling-wave concept” (referring to IEEE Trans. Microwave Theory and Tech., vol. 52, no. 8, pp. 1798-1808, Aug. 2004), a traveling-wave switch configuration is proposed, which integrates additional inductance into an artificial transmission line. This configuration allows integration of the undesirable characteristics into the transmission line, and thus the switch may have a wideband frequency response and good switching characteristics.
Since the undesirable characteristics of the switch are equivalent to lumped elements, U.S. Pat. No. 7,106,146 (entitled “RF Switch”) performs effective impedance matching with these equivalent lumped elements. Accordingly, other techniques have been proposed to replace the elements in a filter with switching elements, so that the filter may assume the characteristic of a single-pole-single-throw switch, as can be found in, for example, “Theoretical and Experimental Investigation of Novel Varactor-Tuned Switchable Microstrip Ring Resonator Circuits”, IEEE Trans. Microwave Theory and Tech., vol. 36, no. 12, Dec. 1988, pp. 1733-1739; “A band-pass filter-integrated switch using field-effect transistors and its power analysis”, in 2006 IEEE MTT-S Int. Microwave Symp. Dig., San Francisco, Calif./USA, 2006; and “New millimeter-wave MMIC switch design using the image-filter synthesis method”, IEEE Microwave and Wireless Component Lett., vol. 14, pp. 103-105, Mar. 2004.
For example, in the above prior art, “A band-pass filter-integrated switch using field-effect transistors and its power analysis”, a quarter-wavelength impedance transformer 12 is used to integrate two single-pole-single-throw traveling-wave switches 14 and 16 into a single-pole-double-throw switch 10, as shown in FIG. 1. Similar integration can be applied to single-pole-five-throw switches, for example, in U.S. Pat. No. 7,106,146, entitled “High Frequency Switch”. However, as limited by the quarter-wavelength impedance transformer 12, the frequency response of the single-pole-double-throw switch 10 cannot be synthesized. This is because the single-pole-double-throw switch 10 must include two single-pole-single-throw switches 14 and 16, and the impedances and frequency responses of the two single-pole-single-throw switches 14 and 16 may affect each other. The impedance transformer 12 may alleviate this influence. Nonetheless, the frequency response of the impedance transformer 12 itself may still influence the frequency responses of the single-pole-single-throw switches 14 and 16. Therefore, the filter function cannot be effectively integrated into the single-pole-double-throw switch 10.